I'm with Iceberg - I thought it took an external USB 2.0 drive. The literature certainly suggests that. And, I recently got a Western Digital external 500gb drive for $80 after rebate for a similar receiver.
The advertizing on the Viewsat, says it'll take an internal hard drive. You should be able to get a modern 500gb IDE/ATA drive for around $70-80, I would think. There used to be a small difference between those designators, but with modern and large drives, the distinction has pretty much vanished.
What the literature does not say you can use, is a SATA drive (serial ATA). Those are newer, and faster, but not supported, at least where I read about the receiver.
Internal drive might seem a little more integrated, sleeker, etc. And if you use it that way, it'd be great. I remove my drive, hook it to my computer, extract the video, and make DVDs. So, the external drive is more convenient for me.
And for use as a "media center" as suggested in one advert, you'll need a way to get video onto your hard drive. So, look into the specs required for the video formats the receiver will play, if it will even play video that it didn't record. That was a major problem with my Visionsat IV-200, 'till I ran across an answer.
The usb is for taking recorded movies off the PVR or firmware updates only. You have to install an internal HDD for PVR options. The serial is the same and the card slot I have no idea. If you would like to have large external hard drives you could try this.